In photos obtained by PEOPLE, the Secret Obsession actress was recently caught wearing a diamond ring on her left hand while running errands in the affluent area of Beverly Hills.
The two have been linked since 2017 after meeting on the set of a film called Changeland.
The couple sparked romance rumors during the movie's filming after being spotted on a double date at Knott's Bery Fram in Buena Park, California. The pair were joined by Seth Green and his wife, Clare Grant.
Months later, Culkin took Song on a PDA-filled vacation to France with his famous goddaughter Paris Jackson. The actor was seen wrapping his arm sweetly around Song while walking through the city streets.
In 2019, the actress gushed about her relationship with Culkin, claiming they understand each other since they both grew up in the entertainment industry.
"It's almost like child actors...don't even have to talk about it," Song said at the time. "You just look each other in the eye, and you nod, and we know."
On Culkin's 40th birthday, The Social Network star wrote a touching message for her boyfriend, expressing her admiration and love for the actor.
"Happy 49th birthday to this magical being," Song wrote in the caption. "I could sit here and write endlessly about how wonderful, kind, loving, genuine, loyal, honest, brilliant and hilarious you are, and how grateful I am that I get to share and do this life thing with you. But firstly, that would take me an eternity and secondly, you won't even see this because you don't ever use Instagram. Hahaha."
She continued, "My unicorn that I never thought could exist, I am luckiest person in the world because I am loved by you."
The couple welcomed their first son, Dakota, on April 5, 2021. They named him after Culkin's late sister, Dakota.
Prior to being engaged to Song, Culkin was previously married to Rachel Miner from 1998 to 2002. This will be Song's first marriage but she's no stranger to Hollywood relationships. She's dated several high-profile figures, including Joe Jonas and Trace Cyrus.